A microwave filter is a two-port network used to control the frequency response at a certain point in a microwave system by providing transmission at frequencies within the passband of the filter, and attenuation in the stopband of the filter. Typical frequency responses include low-pass, high-pass, bandpass, and band-reject characteristics.
Multi-cavity resonator microwave filters are used in communication satellites, particularly those launched into geosynchronous orbit for communications with ground stations. A plurality of filters are used in a typical satellite, each filter able to separate and isolate a specific signal or frequency bandwidth from all of the signals and frequencies transmitted to the satellite. After separation, each signal is amplified to strengthen the signal, whereafter, the amplified signals are transmitted back to ground stations. A single satellite may be equipped with twenty to sixty filters, depending on its mission.
Cavity resonator filters are hollow structures sized to resonate at specific frequency bandwidths in response to microwave signals communicated to the filter structures. The filter resonates using a specific mode dependent upon the geometry of the cavity. Filters which resonate using one mode only are referred to as single mode filters. Dielectric resonators have been introduced into cavity resonator structures, in part to improve output response and reduce the size of the cavity. Cavities with dielectric resonators are often referred to in the art as "loaded" cavities.
One such cavity filter is described in U.S. Pat. No. 5,220,300 issued to Snyder wherein a series of linearly arranged cavities are each loaded with a dielectric resonator. The wall formed between each pair of adjacent cavities is provided with a sized iris (or opening). Each iris provides a means for coupling magnetic energy between adjacent resonators. Further, a tuning screw partially extends into each iris for tuning the iris coupling.
There have also been numerous attempts at building dual mode filters, where either a cavity structure or a loaded cavity structure is designed to resonate using two modes or "dual modes". One such filter is disclosed in U.S. Pat. No. 3,697,898 issued to Blachier et al. The disclosed filter includes an elongated cylinder having planar walls therein to define a plurality of cylindrical cavities. Each cavity is coupled to adjacent cavities via a specifically sized iris formed in the wall therebetween. Dual mode cavity structures have several drawbacks.
For instance, in a communications satellite, a typical desired output from a microwave filter includes a high degree of linearity for the amplitude of the passband frequency range (the desired output) and linearity for the group delay response, in order to minimize distortion in the signal passing through the filter, while maintaining high rejection slopes flanking the filter passband. All dual or single mode filters typically require external equalization to achieve the desired performance. External equalization necessitates the use of ferrite coupling circulators, thus incurring the mass and volume penalty associated with such devices.
Dual mode filters typically require one tuning screw for each resonator to properly tune the modes and one more screw for each interresonator coupling. As readily seen, a fair amount of time is required for proper tuning of each filter in order to get the desired frequency bandwidth output. In general, dual mode filters are less amenable to transfer function control and flexibility. Lower control of the electrical characteristics means more meticulous tuning is required to make the filter meet performance requirements.
It is well known that general transfer function characteristics can be realized by a filter arranged in the canonical form structure of coupled cavity resonators as disclosed in U.S. Pat. No. 4,477,785 issued to Atia. General transfer functions include the elliptic function response for bandpass characteristics and functions having finite transmission zeros.
For an even number of cavities, this canonical form is symmetrical and consists of two identical "halves". Each of the two halves consists of n direct coupled cavities having "series" couplings of the same sign. Each cavity in one half is coupled to a corresponding cavity in the other half by "shunt" couplings of arbitrary sign. Illustrated in FIG. 1 is a schematic diagram of the canonical form of a 2n resonator filter. The series couplings M.sub.12, M.sub.23, . . . M.sub.n,n+1 all have the same sign (positive) while the shunt couplings M.sub.12n, M.sub.2,2n-1, M.sub.n-1,n+2 must be either positive or negative for arbitrary transfer function realization.
In the canonical configuration, the electrical response characteristics of the filter are governed by almost every cavity. Thus, tuning the filter to achieve a desired response is not immediately obvious. What is needed is a filter having predetermined response characteristics dependent upon substructures or subfilters having preset response characteristics.